Two-dimensional dispersion of image electrons on C$_{60}$ thin films on Au(111) and Cu(111)

Two-photon photoemission (2PPE) has been used for many years to investigate occupied and unoccupied electronic states in clean and adsorbate-covered metal substrates. In this report, femtosecond 2PPE is employed to investigate charge transfer across a metal/organic-semiconductor interface and electronic structure in the thin film overlayer. Monolayer films of C$_ {60}$ have been grown using organic molecular beam deposition in ultrahigh vacuum on Au(111) and Cu(111) substrates. Such films represent a model system consisting of a metal/organic- semiconductor interface. Due to slightly different interatomic spacing in the two substrates, the epitaxial C$_{60}$ films grow as C$_{60}(4\times 4)$/Cu(111) or C$_{60}(2\sqrt{3}\times 2 \sqrt{3})$R30$^\circ$/Au(111). These distinct overlayers have previously been established by low energy electron diffraction and scanning tunneling microscopy experiments. By studying angle-resolved 2PPE, dispersion of image electrons in the conduction band along the surface plane can be measured directly. The fact that the C$_{60}$ overlayer is rotated by 30 $^\circ$ in the Au(111) case with respect to the Cu(111) case leads to distinct dispersion characteristics which correspond to different cuts in the two-dimensional band structure of the C$_{60}$ thin film. Application of an \emph{s}-band tight binding model leads to a reasonable quantitative fit.